1. Field of the Invention
The present invention relates to voltage-controlled oscillators (hereinafter referred to as VCO). More particularly, the present invention relates to a VCO including a resonance circuit including a variable capacitance diode, to which a control voltage for controlling an oscillation frequency is applied through a choke coil.
2. Prior Art
FIG. 1 is a circuit diagram showing an example of a conventional VCO. Referring to FIG. 1, a VCO10 includes a resonance circuit 1, an oscillation stage 2, a buffer stage 3, and an output matching stage 4. The resonance frequency of resonance circuit 1 is changed in response to a control voltage Vc which is applied to a control terminal C. Oscillation stage 2 determines an oscillation frequency based on the resonance frequency of resonance circuit 1. Buffer stage 3 amplifies a signal provided from oscillation stage 2, and prevents the oscillation frequency of oscillation stage 2 from varying with load fluctuation. Output matching stage 4 attains matching of buffer stage 3 with a next-stage circuit which is connected to an output terminal p and suppresses higher harmonics.
Resonance circuit 1 includes a variable capacitance diode VD, a coupling capacitor C1, a resonance inductor L1, and a resonance capacitor C2. The cathode of variable capacitance diode VD is connected to one end of coupling capacitor C1, to the connection point of which the control voltage applied to control terminal C is applied through a choke coil L2. The anode of variable capacitance diode VD is grounded. The other end of coupling capacitor C1 is connected to one end of resonance inductor L1, one end of resonance capacitor C2, and one end of a coupling capacitor C7. The other end of resonance inductor L1 and the other end of resonance capacitor C2 are grounded. Oscillation stage 2 includes an oscillation transistor Q1, and the other end of coupling capacitor C7 is connected to the base of oscillation transistor Q1. Further, the base of oscillation transistor Q1 is supplied with a voltage, which is obtained by dividing power supply voltage V.sub.B by bias resistors R1 and R2 connected in series between a power supply terminal B and the ground, as a bias voltage. A capacitor C5 is connected between the base and the emitter of oscillation transistor Q1, and a resistor R3 and a capacitor C4 are connected in parallel between the emitter of oscillation transistor Q1 and the ground. Capacitors C5 and C4 are arranged to provide a Colpitts capacitance, and oscillation transistor Q1, capacitors C5 and C4 and resonance inductor L1 form a Colpitts oscillator, to oscillate at the resonance frequency of resonance circuit 1.
An oscillation output from oscillation stage 2 is provided to buffer stage 3 through a coupling capacitor C8. Buffer stage 3 includes a buffer transistor Q2, which is supplied with the oscillation output of oscillation stage 2 at its base. The base of buffer transistor Q2 is supplied with a voltage, which is obtained by dividing power supply voltage V.sub.B by bias resistors R4 and R5 connected in series between power supply terminal B and the ground, as a bias voltage. Further, the emitter of buffer transistor Q2 is grounded, and the collector of buffer transistor Q2 is connected to power supply terminal B through a choke coil L3 included in output matching stage 4.
Output matching stage 4 includes choke coil L3, a coupling capacitor C10, and an output matching capacitor C11. One end of coupling capacitor C10 is connected to the collector of buffer transistor Q2, and the other end thereof is connected to output terminal P. One end of output matching capacitor C11 is connected to output terminal P, and the other end thereof is grounded. Note that a high-frequency bias capacitors C6 and C12 are connected between power supply terminal B and the ground, and that a high-frequency bias capacitor C3 is connected between control terminal C and the ground.
VCO10 shown in FIG. 1 has respective components mounted on a substrate of alumina or the like. The capacitance value of variable capacitance diode VD included in resonance circuit 1 is changed according to the value of control voltage Vc, so that the resonance frequency of resonance circuit 1 changes, oscillation stage 2 oscillates at the resonance frequency, and that the oscillation signal thereof is provided from output terminal P.
Choke coil L2 provided between control terminal C and variable capacitance diode VD shown in FIG. 1, that is, in a portion to which the control voltage is applied, is for removing noise superimposed on a power supply line or the like. Choke coil L2 is generally formed as a conductive pattern printed on the substrate. Change in length of the conductive pattern forming choke coil L2 causes the impedance of a circuit portion to which the control voltage is applied to change. It is experimentally confirmed that this changes various characteristics of a VCO, such as C/N (carrier/noise) characteristics, S/N characteristics, control voltage sensitivity characteristics, and control voltage-to-oscillation frequency characteristics. Therefore, it is necessary to determine the length of the conductive pattern so that the above-described characteristics take prescribed values, respectively, whenever a VCO different in oscillation frequency is designed.
In order to change the length of the conductive pattern for every VCO different in oscillation frequency, however, it is necessary to prepare many kinds of substrates having different conductive patterns, which in turn increases design cost and management cost. Therefore, from the standpoint of mass production, even if the characteristics are deteriorated more or less, a plurality of kinds of VCOs different in oscillation frequency must be manufactured with one kind of substrate.